Lasers normally have three energy states: the ground state, the upper energy state and the lower energy state. The output power of the laser is proportional to the ratio of the molecule population in the upper state to the population in the lower state. That is, the greater the population inversion the greater the gain and the higher the power output of the laser. The population of the upper state is a function of the pumping of the laser, whereas the population of the lower state is a function of the temperature of the gas. The laser is driven by pumping ground state molecules into the upper state to increase the population inversion ratio. However, when this is done the associated heat drives some molecules from the ground state to the lower state, thereby decreasing the population inversion ratio. Eventually the laser saturates and no increase in output power occurs in response to further increases in pumping power.
One approach to overcome this problem is to pump with very short electrical pulses so that the population of the upper state increases before the heating effect drives a significant population from ground to the lower laser state, thus significantly increasing the population inversion. This achieves laser output pulses up to ten times the power of continuous operation.
An alternative way to create this pulsed effect in a continuous laser is to maintain a continuous electrical discharge in the laser but flow the gas through the discharge region at such a velocity that the gas is only in the discharge region for a short time: the time of the desired pulse width which pumps but does not heat equivalently and so increases the population inversion. While each volume of gas responds as if pulsed, the effect is in fact a continuous laser with ten times the power of slow flow or of a sealed laser.
However, to obtain such high-speed gas flow a high-speed gas pump and a heat exchanger capable of cooling the gas are required. These are expensive, large, noisy, and consume substantial power. But this is a preferred approach in many medium and low-speed pulsed laser systems.
In some applications, pulses of longer duration are required which are too long to obtain the energy conversion without heating the gas and thus decreasing the population inversion ratio yet are not long enough to require continuous high speed gas flow. Thus it is inefficient and expensive to maintain the high-speed pump and heat exchanger continuously for these applications.